A large number of disposable biomedical electrodes for heartbeat monitoring and the like are currently available. Such electrodes are designed to detect variations in the electrical potentials which appear on the skin of a patient and which reflect heartbeat or other electrophysiological activity. Since these skin potentials are very small, e.g., on the order of 2 millivolts, the potentials must be amplified considerably to provide effective outputs reflecting the electrophysiological activity of interest. For this reason, electrodes must have very high performance to minimize noise factors and maximize the quality of the signals transmitted to the testing apparatus. An electrolyte, typically a gel or adhesive film, is generally used to enhance electrical connection between a sensing element of the electrode and the skin.
Electrodes used to obtain electrocardiograms (ECG) should also be able to recover from polarizing overloads. Polarization occurs when an extraneous external voltage is applied to the patient wearing the electrode (e.g. defibrillation voltage) and the electrode is not able to function. An ECG electrode which can recover from the superimposition of a large defibrillation current in a relatively short period of time is important to detecting whether the defibrillation procedure has started the heart. Thereafter the electrode needs to continue to pick up the tiny voltages associated with the heart's beating without loss of fidelity of the signal. The electrode needs to recover to a half-cell potential approaching its original half-cell potential. When the instrument used with the electrode has a very high quality input amplifier, the half-cell potential does not need to recover to the same extent required with lower quality instruments The electrode at a minimum should recover so that its polarization (the difference between its new half-cell potential and its original half-cell potential) is no greater than 300 millivolts. Preferably polarization should be no greater than 100 millivolts after 5 seconds after four defibrillation pulses.
Electrodes exist which are sufficiently non-polarizable. These perform well by virtue of their being highly reversible That is, the chemical reactions which occur when an electric current passes through the electrode are completely and immediately reversible. The electric current itself is composed of the movement of charged ions through the electrolyte which forms the interface between the skin and the sensing element of the electrode. When these ionic currents meet the sensing element, normally a "metallic" conductor, they are converted into electronic currents which are carried by wires, switched, detected, amplified, and so on in instruments such as an electrocardiograph machine. These reactions at the sensing elements are often upset by the superimposition of external voltages, resulting in a polarized electrode.
In the bioelectrode art, a sensing element made from silver/silver chloride has become the standard for high quality electrodes of low polarizability. U.S. Pat. No. 4,377,170 (to Carim) describes one silver/silver chloride nonpolarizable electrode. In electrodes of this type an element body made of (or coated with) silver is coated, either mechanically or by chemical reaction, with silver chloride The electrolyte, e.g., sodium chloride or the like as the ionic-conducting species, is frequently contained within an aqueous gel or as an aqueous solution held in a sponge. This type of electrode, though it can work quite satisfactorily, has two major disadvantages. It has a high cost, because of the need to use silver, and corrosion is a problem, because of the presence of corrosive sodium chloride solution
High cost is partially alleviated by substituting cheaper materials for at least a portion of the silver in the sensing element Such elements made by silver-plating molded plastic are well known. However, the process of accurately and reproducibly depositing a silver coating onto a plastic substrate requires careful control, as does the subsequent conversion of the silver surface to a silver chloride surface. In addition, malfunction of the electrode can occur through chemical reaction between the sensing element and the electrolyte and also through corrosion of other parts of the electrode, such as the metallic snap connector frequently used to connect the lead wire to the sensor.
Sensing elements made of materials other than silver/silver chloride have been used in biomedical electrodes. The use of nonmetallic, noncorrosive sensing elements made of carbon or carbon-containing molded plastics is disclosed in U.S. Pat. Nos. 3,566,860 and 4,109,648. However, these elements are highly polarizable when used with known electrolytes and they are generally unsuitable for use in monitoring electrodes. U.S. Pat. No. 3,976,055 (to Monter et al.}describes an electrode in which an electrically conductive but galvanically inactive sensing element such as carbon-impregnated plastic has one or more metal particles anchored to the surface of the sensing element at the interface between the electrolyte and the galvanically inactive sensing element.